NASA, LF Absorption and Gas Flow Resistivity

[johndykstra]

Really? I thought the ability to measure the thickness of a sheet of paper from 100 miles away was pretty redonkulous.

[Brainchild]

Yeah, that too. Basically scientists are amazing.

[tlindaas]

Hi, I used Soundflow to calculate absorption coefficients for 2 inches of insulation, with a density of 3pcf and a specified gas flow resistivity of 16000, 23000 and 27000 Rayls (16, 23 and 27 kPa*s/m2).

All the coefficients are calculated according to the American ASTM C423 test standard used by Owens Corning (diffuse field, specimen size 8x9 feet.) Exception: For educational reasons, I have included, in the bottom row, the calculated result of using the European ISO 354 standard instead (diffuse field, specimen size 3x4m (approximately 10x13 feet)).

Compare with Owens Corning's measured absorption coefficients for OC 703:

Frequency numbers: 125hz 250 500 1000 2000 4000
As measured by OC: 0.17 0.86 1.14 1.07 1.02 0.98
Soundflow - GFR 16: 0.22 0.65 0.92 0.97 0.97 0.96
Soundflow - GFR 23: 0.24 0.73 0.98 1.00 0.97 0.96
Soundflow - GFR 27: 0.25 0.77 1.01 1.00 0.97 0.96

(GFR27 ISO354 size: 0.25 0.72 0.94 0.97 0.96 0.95)


Apart from the 125hz data, this seem to strenghten the case for a gas flow resistivity of 27000 for OC 703. (From what I've read, measurements become notoriously unreliable when approaching 100hz, so this 125hz anomaly could possibly be disregarded.)

Sadly, it seems impossible to arrive this close to the published absorption data by using Chris Whealy or Stanley Hall's calculators - I've tried. The Hall calculator lacks the possibility to specify dimensions, as do the Whealy calculator. The Whealy calculator also lack formulaes other than Delany & Bazley (I used Miki in Soundflow, which increased the calculated bass and low mid absorption). The Whealy calcuator also lacks the possibility of specifying random incidence/diffuse field (this dramatically increases the calculated bass and low mid absorption.)

The ability to specify finite dimensions of the specimen (as opposed to infinite), combined with the ability to specify random incidence/diffuse field, basically gets Soundflow closer to the reverberation room conditions specified by ASTM C423 and ISO 354. (This is important because almost all insulation material is measured according to these standards).


References:

Dimensional requirements:

ASTM C 423 absorption coefficient measurement standard:
"9.1.1 The specimen shall be a rectangular patch assembled from one or more pieces. An area of 6.69 m2 (72 ft2) is customary and recommended, in a shape 2.44 by 2.74 m (8 by 9 ft). An area less than 5.57 m2 (60 ft2) shall not be used, and extreme aspect ratios, such as long narrow strips, shall be avoided."
Link: http://dc387.4shared.com/doc/AFPeMpJW/preview.html

ISO 354 absorption coefficient measurement standard:
"6.2.1.1 The test specimen shall have an area between 10 m2 and 12 m2. If the volume V of the room is greater than 200 m3, the upper limit for the test specimen area shall be increased by the factor (V/200 m3)2/3. [...] 6.2.1.2 The test specimen shall be of rectangular shape with a ratio of width to length of between 0,7 and 1."
Link: http://ebookbrowse.com/gdoc.php?id=2...eea534fe6ea6bc


EN 13162: General requirements for European insulation, including the publication of data and the use of the ISO 354 standard:
http://bzwxw.com/soft/UploadSoft/new...13162-2001.pdf

ASTM C 522 air flow resistivity measurement standard:
http://bzwxw.com/soft/UploadSoft/new...-C522-2003.PDF


(Some of the links are slow, have patience.)


OC measurement data: http://www.owenscorning.com/comminsu...s700Series.pdf

An article that conclude that the "edge effect" is more important than previously thought:
http://nwaalabs.ipower.com/Files/NWA...s/IN09_091.pdf
http://nwaalabs.ipower.com/Files/NWA...s/IN09_092.pdf


Soundflow 30 day trial: http://www.afmg-support.eu/AFMGRegTr...SoundFlow.aspx

[DanDan]

Great stuff. 27K it is then.
DD

[avare]

Thank you tlindass for increasing the knowledge base of Gearslutz and acousticians across the world.

Modestly and absorptively,
Andre